Multichannel coherent amplitude recording and reproducing system



United States Patent Inventor Appl. No.

Filed Patented Assignee MULTICHANNEL COHERENT AMPLITUDE RECORDING AND REPRODUCING SYSTEM Roy E. Pearson Seattle, Wash. 761 ,019

Sept. 13, 1968 Dec. 15, 1970 the United States of America as represented by the Secretary of the Navy. by mesne assignment 1 Claim, 2 Drawing Figs.

U.S.Cl 179/l00.l, l78/6.6A, 340/l74.l

cu s/0p 179/1001, 0/174.1(B),

[56] References Cited UNITED STATES PATENTS 3,096,483 7/1963 Ransom 331/51 3,433,903 3/1969 .Murray.... 179/1002 3,017,616 1/1962 Runyan 179/1002 Primary ExaminerBernard Konick Assistant Examiner Raymond F. Cardillo, Jr.

AttorneyRaymond l. Tompkins, John W. Pease and John F.

Miller ABSTRACT: The invention avoids coherent phase errors in a multichannel recording and reproducing system by heterodyning the input information channels with heterodyning frequencies derived from a common base frequency. The effects of recorder tape skew, wow, and flutter are eliminated in the reproduced information by demodulating with frequencies derived from a recorded pilot frequency which is affected by tape skew, wow, and flutter to the same degree as thejnfonnation channels. The effects of drift in components such as oscillators, etc. is eliminated by deriving all the heterodyning and demodulating frequencies from a common base frequency using digital techniques.

7 4p: @carar 60 EU DEC] 5197B fin: @carder Roy 5. Pearson INVENTOR.

MULTICIIANNEL COHERENT AMPLITUDE RECORDING AND REPRODUCING SYSTEM BACKGROUND OF THE INVENTION This'invention is in the field of recording and reproducing. Among the many problems encounteredin the prior art have been the reduction of coherent phase errorsin multichannel systems, the reduction of wow and flutter in recorders, and the reduction of frequency drift invarious components. Many attempts have been made ,to solve these problems; however, prior to applicant's invention, only limitedsuecess had been achieved.

SUMMARY OF THE INVENTION digitally derived from the common recorder frequency is recorded on a tape with the heterodyned information.

The term digitally derived" as used herein refers to the use of digital counters, dividers, or equivalent devices, for dividing a common frequency into selected frequencies. The use of digital elements for frequency division assures accuracy and stable relationships between'frequencies. When the recorded information is reproduced, the recorded pilot frequency is reproduced to serve as a common frequency from which a plurality of demodulating frequencies are digitally derived. The information channels are demodulated with a respective demodulating frequency. The use of digitally derived frequencies eliminates the effect of drift in circuit components. Since the reproduced pilot frequency and derived demodulating frequencies have been affected by tape skew,'wow, and flutter to the same degree as the reproduced information, the demodulated information is free of these effects.

BRIEF DESCRIPTION OF THE DRAWING FIGS. I and 2 comprise a block diagram showing the structural and functional interrelationships of the principal elements of the invention.

DESCRII'TION OF THE PREFERRED EMBODIMENT The principles of the invention are illustrated in the drawings. In FIG. 1 four information input channels, 1, 2, 3, and 4, furnish signal frequencies f,,' f f and f, to be recorded. These frequencies are amplified in respective amplifiers 10, 12, 14, and I6, and then mixed in respective full wave balanced mixers 20, 22, 24, and 26, with respective heterodyning frequencies f f,,, f an'df,,. Mixers 20, 22, 24, and 26 are phase stable ring modulators which reject all harmonies produced by heterodyning by a factor of 60 db. The subcarrier output or difference frequencies from mixers 20, 22, 24, and 26 are amplified by respective amplifiers 30, 32, 34, and 36, and filtered by respective'subcarrier filters 40, 42, 44, and 46 to eliminate the undesired output frequencies from the mixers. The remaining subcarrier frequencies f f, f 12,, j; j}, and f1, f, are then forwarded through respective emitter follower circuits 50, 52, 54, and 56m be recorded in tape recorder 60. i g

The hcterodyning frequencies 15,, f, j}, and f are derived from reference frequency generating circuits in recorder 60, and are selected so as toheterodyne the information signal frequencies into a 40 kc band centered at 2l6 kc on a single track in tape recorder 60. Recorder 60 may be a commercially available recorder having built-in precision frequency generating means, for example a Honeywell Model 7700 Recorder having a 3.456 mc crystal controlled oscillator and several precision countdown frequency outputs for the control of external systems. Since the heterodyning frequencies are derived from a single source, phase errors between channels between the channels fall within the rejection band of filter 42. Recorder 60 contains a crystal controlled 3.456 mc oscillator and count down circuits (not shown), for developing a plurality of precise frequencies. Two of the heterodyning frequencies, f b and f,, are obtained. directly from these frequency generating circuits in recorder 60' over the lines shown. These are connected to mixers 22 and 26 through respective limiter amplifiers 72 and 76. Heterodyning frequency f is obtained by dividing a frequency from recorder 60 in a divide-by-ten feedback counter 84, and forwarded through limiter amplifier 74 to mixer '24. I-Ieterdyning frequency f}, is derived by dividing a recorder frequency in a divide-by-nine feedback counter 80 and a cascaded divide-bytwo square wave circuit 81. Frequency j}, 'is forwarded to mixer'20 through limiter amplifier 70, Limiter amplifiers. 70, 72, 74, and 76 provide isolation and low'impedance drive for mixers 20, 22, 24, and 26. The exact frequency dividing arrangement may vary depending on the frequencies available in the recorder employed, as long as the heterodyning frequencies originate from a single source. Heterodyning frequency f, is filtered in a filter 90, amplified in amplifier 91, and recorded in tape recorder 60, as pilot frequency f,,. As pilot frequency f,, is recorded, it is modulated in accordance with the skew, wow, and flutter characteristics of recorder '60, as are the subcarrier frequencies f, fi f f,,, etc., previously described.

Now, referring to FIG. 2, when the information recorded in recorder 60 is reproduced, the information in channels 1-4 goes through respective delays 21, 23,. 25, and 27 to respective ring demodulators 31, 33, 35, and 37. The delays 21, 23, 25, and 27 are adjusted to compensate for time lag in frequency sources and filters. After demodulation the information is filtered in respective filters 41, 43, 45, and 47. Ring demodulators 31, 33, 35, and 37 are supplied with digitally derived demodulating frequencies f,,,f,,, j}, and j}, sothe filtered output of the respective channels 1, 2, 3, and 4 is f,, f f and f,,.

The demodulating frequencies are derived by reproducing the pilot frequency f, in recorder 60, filtering and amplifying f in a filter 100 and a limiter amplifier 102 to obtain frequency f,., then multiplying and dividing f .to obtain the other demodulating frequencies. The frequency f, is first multiplied by a factor of ten in multiplier 104, then divided by a factor of eight in the first three stages of a binary counter 106 to obtain the frequency f, The frequency f, is connected to ring demodulator 37 through an emitter follower 108. In a like manner fi,- is obtained by'dividing the frequency 10f, by a factor of 64, using all six stages of binary counter 106. The frequency f}, is fed to ring demodulator 33 through an emitter follower 110. In a like manner f,, is derived from lQfi. by dividing 10f by a factor of nine in a counter 112, then dividing the output of 112 in a divide-by-two circuit 114. The output of 114, f;,, is fed to ring modulator 31 through a limiter amplifier 1 16. A ring modulator may be used as a modulator, demodulator, or phase detector, and. may comprise four diodes or equivalent elements connected in a ring shaped structure in such manner that current can flow easily in one direction around the ring and is prevented from flowing in the opposite direction by the high impedance of the diodes. Input and output connections are made at the four nodal points of the ring. Ring modulators are described at, for example, page 432 of the text Modem Dictionary of Electronics. by Graf, published by theBobbs-Merrill Co., Inc. of New York. Ring modulators are used in the present invention because they re- .ject or greatly attenuate all of the harmonics produced by the when the reproduced information frequencies f, f fl fb, etc., in channels 1-4 are demodulated with the reproduced frequencies, the skew, wow, and flutter effects imposed by recorder;6 will be removed. Therefore, the output information in channels 1-4 will be f f f and f the same as the input information.

' Means fonresetting the counters 80, 81, 84, 106, 112, and 114 are shown-as a one-shot multivibrator 120 and the conriecting reset lines shown. A start switch 8, is connected to the one shot multivibrator 120. The arrangement is such that when the apparatus is started, all the counters are reset.

Since all the heterodyning and demodulating frequencies are digitally derived from a sync frequency in recorder 60, the effects of drift in circuit components 'such as oscillators, etc, are eliminated.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. In a multichannel recording and reproducing system hav-,

ing a plurality of information channels, the improvement comprising:

a plurality of ring modulator mixing means, one for each respective channel, means for deriving a plurality of heterodyning frequencies from a common base frequency, said heterodyning frequencies being mixed with the inforrnation'in the respective channels in said mixing means, a plurality of subcarrier filter means connected to the output of each respective mixing means to obtain a plurality of difference frequencies and to eliminate cochannel interference from adjacent channels, means for recording the said difference frequencies, means for deriving a pilot frequency from said common base frequency, means for recording said pilot frequency, rneansforf reproducing said difference frequencies and saidpilot frequency, tandem connected digital multiplying and dividing means for simultaneously deriving a plurality ofdemodulating frequencies from said reproduced pilot frequency, and ring demodulator means for demodulating said reproduced difference frequencies with respective ones, of said demodulating frequencies, said means for deriving said heterodyning frequencies comprising aplurality of digitalcounters for dividing said common base frequency. a 

